Stainless steel alloys, especially those of the high chromium-nickel types, are commonly used for components employed in nuclear reactors due to their well known resistance to corrosive and other aggressive conditions. For instance, nuclear fuel, neutron absorbing control units and neutron source holders are frequently clad or contained within a sheath or housing of stainless steel of Type 304 or similar alloy compositions. Many such components, including those mentioned, are located in and about the fuel core of the nuclear reactor where the aggressive conditions such as radiation and temperature are the most rigorous and debilitating.
Solution or mill annealed stainless steels are generally considered to be essentially immune to intergranular stress corrosion cracking, among other means of deterioration. However, stainless steels have been found to occasionally degrade and fail due to intergranular stress corrosion cracking following exposure to high irradiation such as is typically encountered in service within and about the fuel core of water cooled nuclear fission reactors. Such irradiation related intergranular stress corrosion cracking failures have occurred notwithstanding the stainless steel metal being in the so-called solution or mill annealed condition, namely having been treated by heating up to within a range of typically about 1,850.degree. to 2,050.degree. F., then rapidly cooled as a means of solutionizing carbides and inhibiting their nucleation and precipitation out into grain boundaries.
Accordingly, it is theorized that high levels of irradiation resulting from a concentrated field or extensive exposure, or both, are a significantly contributing cause of such degradation of stainless steel, possibly due to induced hardening and/or by promoting segregation of impurities therein. One theory suggested is that irradiation damages the crystalline structure of stainless steel causing vacancies therein which facilitate the rate of diffusion of impurities or trace elements such as phosphorus and silicon and their migration to grain boundaries.
Austenitic nickel-based alloys, moreover, appear to exhibit the same sensitivity to irradiation and in turn susceptibility to intergranular stress corrosion cracking as austenitic stainless steels.